In 1970s, an American Hal Wing designed and manufactured a telescopic multi-purpose ladder, which was commonly known as “Little Giant”. Through years of development, Little Giant ladders have taken the lead in the industry and are popular around the world.
A key technique of Little Giant ladders is the jointing part, called thick plate hinge that joints the upper ladder section and a lower ladder section. Please see FIG. 1-4. The thick-plate hinge 10 comprises an inner plate 11 and outer plates 12 that are folded on outer sides of the inner plate, the inner plate 11 and outer plate 12 are relatively rotatable; the thick-plate hinge 10 further comprises a positioning shaft sleeve 13 designed to lock up and position the inner plate 11 and outer plates 12, wherein, the positioning shaft sleeve 13 has a shaft 14 and a lock pin 15, the outer plate 12 has a stop stub 16 fixed on it, and the stop stub 16 butts against an outer arc surface 110 of the inner plate 11 when the relative rotation angle between the inner plate 11 and the outer plate 12 reaches 180°. The stop stub 16 can improves the bearing performance of the thick-plate hinge 10.
Via the positioning shaft sleeve 13, the inner plate 11 and outer plates 12 of the thick-plate hinge 10 can be locked up when they rotate to a preset angle in relation to each other, for example, the inner plate 11 and outer plates 12 can be locked up via the positioning shaft sleeve 13 when the unfolding angle is 0°, 36°, or 180°. With the structure described above, two thick-plate hinge 10 can be connected between an upper ladder section and a lower ladder section, so that the ladder can be transformed into a double-sided ladder (with 36° unfolding angle) or a long straight ladder (with 180° unfolding angle), to attain multiple purposes.
When the ladder is transformed into a straight ladder, both the inner plate and the outer plate of the two thick-plate hinges are unfolded to 180°. When tested in unexpected state or under specific conditions, e.g., tested as per the ladder standards of USA or that of Australia, the tested ladder must be able to bear the test load in a case that either thick-plate hinge work in fail state (the lock pin 15 is drawn out of the lock pin hole). In the prior art, in case either thick-plate hinge in fail state, though the thick-plate hinge still has some bearing capability obtained from the butting of the stop stub 16 and outer plate 12, the stop stub would have displacement on the outer arc surface, since the bearing surface of the inner plate that butts against the stop stub is arc surface; consequently, the bearing angle between the upper ladder section and the lower ladder section is greater than 180°; in addition, the higher the load is, the greater the displacement would be, and it lead to larger relative rotation angle between the inner plate and the outer plate. As a result, the overall bearing capacity of the ladder would be degrade, and the entire ladder is more possible to fail in the test. In that case, the solution in the prior art is to increase the strength of the thick-plate hinge or the strength of the entire ladder to improve the bearing capacity of the entire ladder, which would inevitably increase the cost of the entire ladder severely.